1. Field of the Invention:
This invention relates to a video camera or a still video camera and more particularly to a monofocal lens or a zoom lens adapted for the photo-taking system of the camera of the above-stated kind.
2. Description of the Related Art:
The use of a solid-state image sensor such as a CCD for the video camera has enhanced the sensitivity of the camera. As a result, the diaphragm of the camera must be greatly stopped down in taking a picture of an object which is either under a clear sky or has a high degree of luminance. Under such a condition, therefore, the camera tends to experience hunting of the diaphragm or to be affected by the diffraction of light due to the small aperture. This deteriorates the image forming performance of the camera. To solve this problem, it has been practiced to provide a filmy ND (neutral density) filter which is either disposed adjacent to the diaphragm or at the diaphragm blades.
This is because, the quantity of a bundle of rays passing through the ND filter decreases when the diaphragm is stopped down. Compared with a condition of using no ND filter, the use of the ND filter allows the diaphragm to be in a larger aperture position for the same brightness of the field, so that the adverse optical effect due to a small aperture can be moderated by the ND filter.
In the case of a zoom lens or the like, however, the diaphragm which determines the F number of the lens is disposed at an afocal light flux part in most cases. In addition to that, the reflection factor of the surface of the solid-state image sensor has become higher. Therefore, when the ND filter is mounted on the lens, a ghost image is generated by a bundle of rays L2 produced when a bundle of rays which is first reflected at the solid-state image sensor surface 2 is again reflected by the ND filter surface 3' as shown in FIG. 2 of the accompanying drawings. The same problem also arises with a wavelength selective filter or the like other than the ND filter disposed adjacent to the diaphragm. Even in the case of a monofocal lens, image deterioration might also take place depending on the state of the bundle of rays obtained at a point where the diaphragm is disposed.
In connection with the generation of the ghost by the surface of an image forming lens, a photo-taking lens which is arranged to lessen the possibility of generation of the ghost has been disclosed in U.S. patent application Ser. No. 301,951, filed on Jan. 26, 1989.